Base analogs are derivatives of the normal DNA bases, which may mimic to varying extents the properties of the normal bases. As such, they have the ability to interfere with normal nucleotide metabolism and exert a variety of toxic and mutagenic effects. One example of their application as a toxic compound is usage as antiviral or antitumor agents. Our main interest in base analogs centers around their properties as mutagens, and we use them as probes for studying the various ways by which cells either make mutations or try to avoid them, using the bacterium E. coli as a model system. Specifically, we use purine analogs such as 2-aminopurine (AP), 6-hydroxylaminopurine (HAP), and 2-amino-6-hydroxylaminopurine (AHAP), and the pyrimidine analog P-nucleoside (a bicyclic cytosine derivative) to investigate (i) the mechanisms by which these analogs are converted to mutationally active forms (e.g., a modified dNTP) and (ii) the protective mechanisms that cells use to avoid or diminish analog-induced mutagenesis. Secondly, we are interested in naturally occurring base analogs, which have gained attention as possible contributors to mutagenesis. One well-known example is the oxidation product 8-oxodGTP, which has been shown to be, under certain conditions, a highly mutagenic contaminant of normal cellular dNTP pools. The basic approach in our studies is a genetic one, in which analog-induced toxicity or mutagenesis is studied in a variety of E. coli genetic mutants. These mutants are (i) affected in established pathways of DNA replication, repair, or nucleotide metabolism (to delineate the role of these systems) or (ii) newly isolated mutants generated on the basis of their altered response to these agents (to discover novel pathways for analog toxicity or mutagenesis). The most striking discovery has been the identification of a hitherto unknown detoxification pathway for N-hydroxylated bases, which employs enzymatic activities requiring the molybdenum cofactor. The enzymatic mechanism underlying this molybdenum cofactor dependent activity is being investigated.